Morphological evidence indicates that aging in the brain is associated with regression in dendrites of some cells and growth in dendrites of others, whereas in senile dementia only regression is observed. Biochemical measurements of sialic acid content tend to confirm the loss of axodendritic processes in showing a 20 % greater loss in senile dementia than in aging. We plan to develop additional biochemical criteria to establish whether membrane pathology is involved in aging and in senile dementia. These criteria could be of major assistance in further studies of pathological and aging processes by light microscopic and ultrastructural methods. Our method of developing these criteria is to prepare antibodies against highly purified synaptic membrane fractions from human brain, then to determine which antigenic components of the membrane react with the antibodies, and finally to use the antibodies as reagents to measure the quantity of the various antigens that are present in various brain regions obtained post mortem from mature adults, aged persons without dementia, and patients with senile dementia. These measurements will serve as indices of the loss of axodendritic processes and possibly of synaptic contacts as well.